Document handling devices are presently employed in a large number of different applications. Some examples of such document handlers are devices for counting checks, punch cards, food stamps, paper currency and bank coupons, to name just a few. The basic requirements of such devices are to provide reliable operation at relatively high operating speeds in the feeding, separating, endorsing, and stacking of the documents being handled. For example, during such high speed operations, a separation between documents may be quite small, requiring a sensing device capability of detecting such small separation distances. Due to the high speed operation, it is also important to provide means for protecting against faulty operation since if a jam were to occur with the documents fed at reasonably high rates of speed, severe damage to the mechanism, as well as the sheets, may occur before corrective measures can be taken.
It is also well known that the document feeders providing such high speed operation generate an appreciable amount of dust which, together with other ambient conditions, serves to deteriorate the sensitivity and hence the capability of the sensing means to function properly.